Null steering or adaptive noise cancelling is a procedure which has been known for many years and is described, for example, in such typical articles as "Adaptive Antenna Systems", by B. Widrow et al., Proceedings of the IEEE, Volume 55, No. 12, December 1967, and "Adaptive Noise Cancelling: Principles and Applications", by B. Widrow et al., Proceedings of the IEEE, Volume 63, No. 12, December 1975. In general, null steering is a technique whereby two or more antenna signals are weighted and summed together to form a composite antenna pattern. The pattern is formed in such a manner as to create antenna pattern nulls in the directions of the jamming signals and lobes in the direction of desired signals. Using null steering techniques, nulls on the order of 50 dB below the pattern maxima can be automatically steered in the directions of jamming signals.
Using, for example, a four channel null steerer, each antenna signal is split into an in-phase component and a quadrature component with a 90 degree hybrid circuit or the like. The two signal components are then weighted and summed together along with the signal components from the other antenna weighters, in a final summing circuit. By using a 90 degree hybrid circuit and weighters, a single phasor (any specific signal on an antenna can be represented by a phasor) on a particular antenna can be shifted to any new phase and amplitude desired. If a jamming signal or any other undesired signal is presented on two antennas, for example, the null steerer will shift the two signals (phasors) such that they are of equal amplitude and opposite phase. When these two weighted signals are then summed together in the final summing circuit, they will cancel, thereby forming an antenna pattern null in the direction of the jamming signal. The process is similar when the jamming signal is present on all four antennas. The number of independent nulls that can be formed is equal to N-1 where N is the number of antennas.
The value of each weight is automatically adjusted by feeding back the output of the final summing circuit to a correlator or mixer, which mixes the summing circuit output with the signal component from the corresponding antenna, which is non-weighted, thereby creating a correlation voltage. This correlation voltage is integrated and used to drive the specific weighter for that antenna component. The weighters are always driven in such a manner as to minimize the feedback signal. When the feedback signal is completely eliminated, corresponding to forming a complete null, the output of the correlator is zero and the system has fully adapted. A null steerer implemented in this manner will null out all signals as long as the number of signals is equal to or less than N-1.
In one commonly used technique for preventing nulling of desired signals, a reference signal is used. U.S. Pat. No. 4,079,379, entitled "Null Steering Apparatus For a Multiple Antenna Array", issued Mar. 14, 1978, and assigned to the same assignee, describes apparatus for providing such a reference signal. A copending application entitled "Adaptive Antenna Lobing on Spread Spectrum Signals At Negative S/N", Ser. No. 877,133, now U.S. Pat. No. 4,152,702, Feb. 13, 1978 and assigned to the same assignee, also describes apparatus for providing such a reference signal. However, this apparatus actually transmits two RF signals 90 degrees out of phase. A technical report from the Ohio State University Research Foundation, entitled "An Adaptive Array For Interference Rejection in a Coded Communication System", by K. L. Reinhard, dated May, 1972, discloses apparatus for producing such a reference signal from the output of the processor. The major difficulty with this apparatus is the fact that the reference signal is not in phase with the desired signal and, therefore, the operation of the null steering apparatus lags the reception of the desired signal resulting in a reduction of the signal to noise ratio in the waveform processor.